In the course of incineration of waste materials of various origins, the mixture of waste materials (liquids, sludge, solids) is burned in an incinerator, generally a rotary kiln, under a gaseous current. This produces clinker and fine particles of fly ash (abbreviated herein as FA) and acid gases carrying heavy metals and metalloids. After the clinker and fly ash are removed, these gaseous wastes are made to react with excess lime (or products containing lime) and the resultant treated smoke is sent through bag filters (or dust extractors). These dust filters collect the smoke purification waste materials (abbreviated herein as SPW) which is composed of a very fine, flour-like product.
SPW substantially contains chlorides, lime that has not reacted during purification in the form of CaCO.sub.3 and Ca(OH).sub.2, sulfate, and unburned materials, including fairly substantial quantities of heavy and toxic metals such as lead, copper, zinc, cadmium, iron, aluminum, etc. When they are placed into contact with water, SPW forms a highly basic pH because of its richness in lime. It tends to solubilize due to atmospheric influences and acidic conditions and to produce pollution due especially to leaching of toxic metals. Due to the high content of chlorine and heavy metals, insolubilization is very difficult.
These waste materials, or SPW, are the raw materials which are treated according to the present invention and are distinctly different in composition from other gaseous waste products such as fly ash, electric arc furnace dust waste or desulfurization waste product from coal, oil or lignite thermal power plants.
While the content of SPW will vary depending on the material being burned from day to day, it generally has a chlorine content of between 15 to 30%, a calcium oxide content of between 15 and 30%, a total sodium and potassium content of 0.5 to 8%, less than 2% sulfur and 0.1 to 2% metals including heavy metals. In this alkaline environment the metals are precipitated in salt form.
Processes for insolubilizing waste materials containing SPW have been proposed and consist essentially of treatments with sodium silicate, Portland cement or mixtures of said products. The products obtained using such treatments are highly basic (pH generally greater than 12) and they tend to be ineffective at fixing metal elements, especially those such as lead and zinc. Moreover, hydrogen can form in an untimely manner according to the following reaction: EQU OH.sup.- +H.sub.2 O+Al.fwdarw.Al.sub.2 O-+3/;2 H.sub.2
The hydrogen released may cause an explosive hazard and given the highly alkaline conditions, any explosion would spread corrosive materials.
This product contains an additional problem as well because the waste materials are generally entrapped within the material rather than being part of the solid mass. As a result, the wastes may be viewed as weakening impurities in the cement which may be easily released into the environment.